Field of the Invention
The present invention concerns diffusion imaging with the use of a magnetic resonance (MR) system, and in particular to a method for local correction of gradient non-linearities with diffusion imaging with the use of an MR system. The present invention also concerns an MR apparatus and a non-transitory, computer-readable data storage medium that implements such a method.
Description of the Prior Art
Diffusion imaging with the use of an MR system is currently an indispensable method in medical diagnostics. One established application is determining trace-weighted MR images in which direction-dependent diffusion effects are averaged such that the result is rotation invariant. On the basis of these trace-weighted MR images, apparent diffusion coefficients (ADCs) can also be calculated, which correspond to the rotation-invariant trace of the diffusion tensor divided by its dimension. These are parameters independent of the MR protocol and are becoming increasingly more clinically important.
The correction of undesirable influencing variables is very important in the case of ADC quantification. One fundamental influencing variable is systematic spatial variation of the diffusion gradient fields, in particular non-linearities due to the design lead to location-dependent diffusion weightings and directions, which uncorrected result in the generation of incorrect trace-weighted MR images.
For immediate calculation of the ADCs based on three diffusion directions (trace imaging), there are studies, which are directed toward correction of the ADC values without finding an optimally ideal averaging of diffusion-weighted MR images for generating an approximated trace-averaged MR image in advance. The article “Analysis and Correction of Gradient Nonlinearity Bias in ADC Measurement” by Malyarenko et al., Magnetic Resonance in Medicine, Vol. 71, No. 3, pp. 1312-1323 (2014) describes the approximate correction of gradient non-linearities when determining the ADC in the case of trace weighted imaging with three (nominal orthogonal) diffusion directions.
The article “Analysis and Generalized Correction of the Effect of Gradient Field Distortions in Diffusion-Weighted Imaging” by Bammer et al., Magnetic Resonance in Medicine, Vol. 50, pp. 560-569 (2003) describes the exact correction of gradient non-linearities when determining the ADCs based on a tensor reconstruction, for which at least six (non-collinear) diffusion directions as well as at least one further scan with different diffusion weighting are necessary.
DE 10 2010 029 932 A1 describes a method for correction of image distortions, which occur during acquisition of diffusion-weighted MR images of an examination object, wherein a reference image is acquired by means of a second acquisition process.
Current MR systems for diffusion imaging ignore these problems and assume ideal linear diffusion gradients, and this leads to systematic errors, increasing with distance from the isocenter, when determining trace-weighted MR images and ADC maps derived therefrom.